The present invention relates to an intake system of a multi-cylinder internal combustion engine.
Particularly in a gasoline engine, in order to obtain a high output power of the engine by increasing the volumetric efficiency when the engine is operating at a high speed under a heavy load, the shape of an intake port is so constructed that the intake port has as small a flow resistance as possible. In the case wherein the intake port has such a shape, since a considerably strong turbulence is spontaneously created in the combustion chamber of the engine when the engine is operating at a high speed under a heavy load, the burning velocity is sufficiently increased. However, when the same engine is operating at a low speed, a satisfactory strong turbulence is not created in the combustion chamber, thus resulting in a problem that a sufficient increase in the burning velocity is not obtained.
As a method of creating a strong turbulence in the combustion chamber when an engine is operating at a low speed, there is a method of compulsorily creating a swirl motion in the combustion chamber by using a helical shaped intake port or by using a shroud valve. However, in the case wherein such a method is adopted, since the flow resistance which the mixture fed into the cylinder is subjected to is increased, there occurs a problem in that the volumetric efficiency is reduced when such an engine is operating at a high speed under a heavy load. Consequently, in order to increase the burning velocity when such an engine is operating at a low speed while ensuring a high volumetric efficiency when the engine is operating at a high speed under a heavy load, it is necessary to form each intake port so that it has as small a flow resistance as possible and, at the same time, to create a strong turbulence in the combustion chamber when the engine is operating at a low speed.
In addition, as a method of improving combustion when an engine is operating at a low speed under a light load, there is a method of promoting the vaporization of fuel, in addition to a method of creating a strong turbulence in the combustion chamber. That is, when an engine is operating at a low speed under a light load, the velocity of air flowing in the venturi of the carburetor is low. Consequently, since the relative speed between the fuel injected from the fuel nozzle and the air flowing in the venturi is small, it is impossible to fully atomize the liquid fuel into fine particles. As a result of this, a large amount of the fuel is fed into the cylinder in liquid form, and thus, there occurs a problem in that good combustion cannot be obtained.
In order to avoid the above-mentioned problems, an engine has been proposed in which the intake passage comprises a main intake passage having a relatively large cross-section, and auxiliary intake passages, each having a relatively small cross-section. Each of the auxiliary intake passages opens into a respective intake port defining a part of the main intake passage. In this engine, the mixture is fed into each combustion chamber from the corresponding auxiliary intake passage via its intake port when the engine is operating under a light load, while the mixture is fed into each combustion chamber from the main intake passage via its intake port when the engine is operating under a heavy load. As mentioned above, in this engine, it is intended that the flow velocity of the mixture be increased for promoting the vaporization of fuel by feeding the mixture into the combustion chamber via the auxiliary intake passage having a relatively small cross-section when the engine is operating under a light load. However, at the time of the intake stroke when the engine is operating under a light load, the amount of the mixture which is actually fed from the auxiliary intake passage into the intake port and then into the combustion chamber is extremely small. This is because, since the intake manifold has a relatively large volume, a part of the mixture located in the intake manifold is also sucked into the combustion chamber via the intake port at the time of the intake stroke. In addition, since the intake port of a given cylinder which is in the intake stroke is in communication with the intake ports of the remaining cylinders, fuel-air mixture is also sucked into the combustion chamber of said given cylinder from the auxiliary intake passages of said remaining cylinders via the intake manifold when said given cylinder is in the intake stroke. As mentioned above, since the mixture located in the intake manifold and the mixture fed into the intake ports of said remaining cylinders from the corresponding auxiliary intake passages is fed into the combustion chamber of said given cylinder which is in the intake stroke, the amount of the mixture fed into the combustion chamber of said given cylinder from its corresponding auxiliary intake passage becomes small. As a result of this, since the mixture cannot flow at a high speed in the auxiliary intake passage which opens into the intake port of said given cylinder, it is impossible to sufficiently promote the vaporization of fuel in the auxiliary intake passage of said given cylinder. In addition, since the intake manifold, that is the part of the main intake passage located downstream of the throttle valve of the carburetor, has a relatively large volume, the amount of fuel adhering onto the inner surface of the intake manifold is large. Consequently, when the level of vacuum in the intake manifold is rapidly increased, as in the case wherein an engine is decelerated, a large amount of the fuel adhering onto the inner surface of the intake manifold is instantaneously vaporized. As a result of this, since an excessively rich mixture is temporarily fed into the cylinder, there occurs a problem in that the amount of unburned HC in the exhaust gas is increased. Finally, in a cross-flow type engine, even if the construction of the intake system becomes complicated, no difficulty arises with regard to the arrangement of the intake system because there is a large space around the intake system. However, in a counter-flow type engine, since there is not a large space around the intake system, there occurs a problem as to how to arrange the intake system when it becomes complicated.